1. Field of the Invention
This invention relates to method and apparatus for detecting low level faults in a direct current system and more particularly to method and apparatus for measuring the rate of current rise to detect low level short circuit current in a mint trolley system.
2. Description of the Prior Art
In many electrical systems the magnitude of a fault current associated with a short circuit is several times that of the normal load current. This is not the case for most coal mine trolley systems where the fault current might be as low as 125 percent of the normal load current. Because of the low current level of the faults, the circuit breakers may not detect the faults. In underground mining operations a direct current trolley system is utilized for the transportation of men, supplies, and coal from one location to another in the mine. In addition, the trolley system serves as a power source for stationary loads, such as lights, pumps, and heaters.
In a trolley system direct current circuit breakers must be set sufficiently high to allow tracked vehicles, such as locomotives moving trains of coal cars, to pass by the circuit breaker without tripping the circuit breaker. Also, modern track haulage equipment demands high current, for example, 3,000 amperes dc, and as a result rectifiers must often be tied together to meet these high current demands. A typical "coal train" consists of thirty to forty-five 20 ton coal cars and two 50 ton locomotives. A 50 ton locomotive typically uses four 200 horsepower series wound direct current traction motors.
Unlike most electrical power distribution systems, the magnitude of a short circuit current in a mine trolley system ranges between 125 and 150 percent of the expected system load current as opposed to several times that of the normal load current for a electrical power distribution system. The standard relay accuracy of known electro-magnetic overload relays is plus or minus 15 percent, allowing only a 10 percent safety factor. In addition fault current values cannot be guaranteed because of system degradation due to moving vheicles derailing and damaging feeder wires or rail bonds, acidic water attacking electrical conductors, and man-made splices loosening with age.
The major components of a direct current mine trolley system are the trolley/feeder wire system, the track/return feeder system, direct current rectifiers, and bore hole feeder cables. In many cases the trolley wire system may extend 20 miles in length. Trolly feeder wire, if used, is conventionally made of copper or aluminum. One or more feeder wires may be placed in parallel with the trolley wire. The voltage of a conventional trolley system is 300 or 600 volts dc. Insulated trolley wire hangers connect the trolley wire and the trolley feeder wire together. This parallel combination of the trolley wire and the trolley feeder wire serves as the "hot" conductor of a dc trolley system.
The size and weight of the trolley rails used in a mine is dependent on the type and weight of the load being transported along the rail system. Conventionally each rail is 30 feet in length. The rails are joined by a bolted connection and a "bond" is welded across each joint to assure a good electrical connection. Both rails are "cross-bonded" together at 200 foot intervals to provide an electrical connection between the two rails. Occasionally, a return feeder wire is run in parallel with the track system to increase the overall current carrying capacity and to provide a continuous electrical connection to the mine rectifier. It is also well-known that the mechanical connections between the rails can become loose and the rail bonds and crossbonds damaged by derailments of track-mounted equipment. The track and return feeder wire are also connected together at intervals to serve as the "return" conductor of the direct current trolley system.
It is the conventional practice to supply an underground mine with incoming three phase alternating current voltage, in the range between about 2,300 to 12,470 volts ac, which is converted by mine rectifiers to either 300 or 600 volts dc at a power rating of 300, 500, 750, or 1,000 kw. Rectifiers are selectively located in the mine based upon their capacity and need. It is a conventional practice to install rectifiers near the track entry for efficient connection to the rail and to the trolley. Large mines require large rectifiers which generally are located on the surface and are connected to the trolley wire system by means of bore hole cables. The length of the cable depends upon the depth of the coal seam. Generally there are two "hot" and two "return" conductors per cable. In this manner the current carrying capacity of the cable is increased and the voltage drop across the cable is minimized.
Thus with the above described modern trolley system used in an underground mine numerous trolley system/load combinations exist. These combinations, particularly at the load levels encountered, make it difficult for the known overload detection means to distinguish between a low current fault and a high current legitimate load on the trolley system. Upon the occurrence of an arcing fault, the increased system resistance and/or decreased driving voltage may not allow the necessary fault current to flow, allowing the fault to go undetected.
Therefore, there is need for method and apparatus for detecting high resistance (arcing) fault magnitudes at least significantly less than the circuit breaker static trip threshold. The apparatus must be easily connected to the trolley system and nuisance tripping must be minimized. This requires consideration of a number of factors relating to the trolley system, such as the type and size of the trolley system, the trolley system voltage, and the location of insulators for electrically insulating one section of a trolley system from another.
While it is well known to utilize circuit breakers to provide protection of coal mine trolley systems, the size and speed of the track haulage equipment has increased to the point where current magnitude alone cannot be used to distinguish between a system fault and a legitimate trolley system load. A low level fault detection system is necessary for fault levels less than 300 amperes where the necessary circuit breakers are tripped but legitimate 5,000 ampere trolley system loading is allowed to occur. Therefore, there is need for apparatus operable to distinguish between a low level fault and legitimate trolley system loads.